High-frequency signaling system



Nov. 25, 1-930. F: w. FRINK HIGH FREQUENCY SIGNALING SYSTEM 2 Sheets-Sheet 2 Filed Sept. 27, 1928 Patented Nov, 25 1930 r j UNITEDII- TATE by FR DE K W- RINK,. F CHE KNEW .YOBKQAssrQNbn To e me 311m PATENT IC .TEICHCOM.PANY: A OR R T ON QFJ WH Y R emm mgmvqy ,QSIGNALI NG j sirs'rnm i invention relates to high frequency carrier current communication systemsemploying means for repeating signals between the diiferentstationsof the 'system; Still- [more particularly my invention relatesto means for repeating signals between carrier" current stations each otsjwhiclrboth'transmit and receiveat the samejfre'quency, but of ie which certain stationsoperate at different;

signaling systems, and more particularly to frequencies frorn'the others. The equipment at the different stations may ;be of the nature indicated, for example, in Patent No. v

ber 25,1928] 1,696,566; Emmett FfCa'rter, issued Decem In systems in which-"carrier current trans mittedovert-he lines ofa power distribution system are employed for communication purposes the frequency of the current employed is usually-f determined either the chaf acteristics of the line, or by the fact that in dependent carrier channels are desired, "as when the diflerent systems are conducti ely' interconnected. Accordingly carriercurrents for communication purposes upon diiierent power distribution systems, i or upon different lines, or branches, of the same system'may necessarily be of differentfrequencies. Fre-" quently it may be{ desirable i to" effect co1n- V munication between stations located on such different systems; or such diflerentilinesjor brancheso fthe same system, which stations operate at difierent frequencies. When such intei connection betWeen difierent lines or between statlons operat1ngat d fferent "fre- -quencies on the same line 1s desired different eXpedients may be utilized forflaccomplish ing the purpose. Interconnecting circuits be- 1 tween the different lines may, of coursefbe utilized in such amanne'r that all of thelines become a common conductlng channel "for carrier current. If des red a -repeating Sta-- tion oit the type shownan'd described. in ap connected in the interconnecting' circuit beplication- Serial No. 262,581 or WVilli'am A.- Tolson, entitled Signal system, filed March 17, 1928, and which' is assigned toth'e same assignce as thepresent application may be tween the different lines, this station :being' arranged totrepeat the signals-received from be provided.

either line to-the other line atthe frequency at Whichthestations'on the other-line are adapted to operate. Asanother-expedient means, for example, ofthe. type shown in applicat1on,qSerial No. -l92,7 51 of Stanley Clark; entitled High frequency; communicationsystemsfiledflMay 1931927 and which' is assigned; to the same assigneeasthe present application, maybe employed whereby each stationon each line may be arranged to'0p erate'at either of two frequencies. I i

d vhile' either ofthe above expedients offer satisfactory solutions to the problem situations; may ;occur, as for example, where no houslng facillt es aret avallable at the point Wherethe interconnection of the differentsysteins can most economically be made, or

ceived from any of the stations oneither line i may befrepeated to stations on the other'line at i the frequency which stations on the; t

other'line opjerate 1" A further object ofmy invention is' to provide meansin connection with a repeating station whichflmay' be used foil-repeating sig nals between stations on the same line where bysubstantially "the same equipment maybe utilized for repeating signals between stations on the said line and thoseof a separate" line orlbetween the previously mentioned stations on thesame line and other stations which operate at a difierentfrequency. A-furtherobject of my invention isto provide means whereby intercominunication may be had between three or more stations connectedto the same or todifierent lines, at.

least two of which stations, located on the same side of the repeater operate at different "frequencies, and t0 provide'means whereby this may be accomplished with aminimum'of equipment atthe repeating station. While I have particularly mentioned the repeating of signals between different lines of a power distribution system or between different systems it will, of course, be understood that I do not wish to be limited in this respect since the same may be accomplished where stations operating at different frequencies are associated with the same line.

The novel features which I believe to be characteristic of my invention are set forth with particularity in the appended claims; My invention itself, however, both as to its organization and method of operation, together with further objects and advantages thereof may best be understood by reference to the following description taken in connec tion with the accompanying drawing in which Fig. 1 represents a repeating station located upon a transmission line which may be utilized for repeating signals between stations on that line and stations on a second line at a different frequency; Fig. 2 shows in greater detail the circuits which may be employed in connection, with the system shown in Fig. 1, and Fig. 3 represents a repeating station which is adapted to repeat signals located in different directions from the station and between either of these stations and stations on a different line.

. Referring to Fig. 1 of the drawing I have shown at 1 and 2 apair of power distribution systems, both of which have associated therewith carrier communication stations 3and 4. The stations 3 which are located upon the system 1 may operate at the same frequency and at a different frequency from the station, or stations, 4 located upon the system 2. The

two systems 1 and 2 may be interconnected by means of a suitable coupling circuit 5 which is represented in the drawing as a pair of condensers but which may comprise any suitable means offering a low impedance to current of the carrier frequency and a high impedence to current of the power frequency. This interconnecting circuit may be located at some point where the two systems are in close proximity but where it is impractical, as for example, due to the lack of housing facilties, etc., to locate a repeating station whereby the difierent stations 3 and 4 may intercommunicate. In order that these stations may intercommunicate without the neoessity for so arranging the circuit at each of the stations that all of the individual stations may operate at different frequencies, 1 have provided a repeating station 6 which may be located at any point upon either of the lines 1 or 2 and which is so arranged that it will repeat signals received from any of the stations 3 to the stations 4 and vice versa. Since station 6 is arranged to receive signals from the line 1 and to return signals to the same line for transmission in the same direction from which they were received I choose to call the repeater located at the station 6 a reflex repeater.

The repeater at the station 6 comprises the usual balance transformer arrangement, or hybrid coil, 7 which is connected to the transmission line 1 through coupling condensers S and a suitable tuned circuit arrangement 9. Ihis tuned circuit device is so constructed and arranged that it offers a low impedance to currents of the frequencies at which the stations 3 and 4 operate and a high impedance to other frequencies. The hybrid coil 7 is provided with a pair of output conductors 10 to which are connected the input circuits'of a pair of detectors 11 and 12, these input circuits being connected in series. The hybrid coil 7 is also provided with an input winding 13 which is arranged to be energized with currents supplied from the output circuit of an amplifier 14. The input circuit to the detector 11 is tuned to respond to the frequency of stations 3 which are located upon the system l and that of the detector 12 is tuned to respond to the frequency of stations 4 which are located upon the system 2.

The amplifier 14 is arranged to be supplied with high frequency currents from either of two oscillation generators 19 and 26, the oscillation generator 19 being adjusted to produce high frequency current having the frequency to which the stations 4 are adapted to respond and the oscillation generator 26 being arranged to produce current having the frequency to which the stations 3 are adapted to respond. The operationof the oscillation generator 19 is controlled in response to voice currents received from the stations 3 through detector 11 and line amplifier 16 by means of a suitable electron discharge device 17, which is supplied wit-h speech currents from the output circuit of the line amplifier 16. Similarly the operation of the oscillation generator 26 is controlled in response to speech frequencies received from the stations 4 through detector 12 and line amplifier 23 by means of a control electron discharge device which is supplied with speech currents from the output circuit of amplifier 23.

The amplifier 14 is provided with a suitable modulating device 22 which may be of any well-known construction and which is arranged to be supplied with speech currents through amplifiers 18 and 24 from either of the detectors 11, 12.

As thus arranged the operation of the system is as follows: When currents are rcceived having a frequency at which the stations 3 are adapted to operate they are demodulated by the detector 11 and the audio currents detected therefrom are amplified by the amplifier 16 and supplied to the con trol device 17 and through amplifier 18 to the modulator 22. The control device 17 causes the oscillation generator 19 to produce oscillations having the frequency of the station 4, these oscillations being amplified rent cannot flow in the anode circuit to permitfthe production of oscillations by. the;

device 19. The 'grid potential, howeveryoi- V by the device 14 andmodulated by the device 22' in response to the received voice currents.

These currents are then supplied to the line from whichthey are received by the station 1 l. The control device 17 at the same time causes energiZation-of a relay 20, thereby interrupting the output circuit from the-detector'12 and preventing operation of the oscillation generator 26 in response to any currents which might beproduced'in' the output circuit of thedeteotor 12 in response to currents which may be supplied thereto, eitherfrom the amplifier 14] or from the line. Injthe same way currents received from the station tare rectified the device 12 and the audio currents detected therefrom are amplified by the line amplifier 23 and suppliedto control tube 25 and speech am- ,plifi-er24. The'control tubej'in turn causes "the production "of oscillations by thegen erator 26 which are supplied to the amplifier 14L, theseoscillations being modulated by the device 22 and supplied to the line and to the stations The-operation of the control device; 25 similarly causes energization of the relay 27, thereby interrupting thefoutput circuitfromthe detector 11. a

The circuits b means of which the'above and 30 fromeither of apair of oscillation generators26 and 19 respectively. *Thse' oscillation generators are of any suitable construction employing electron discharge devices; a The anode circuit of the*electron discharge device 19 extends from the anode of that device through a source of potential 31,resistance 32,space between the cathodecandanode ofcontroldevice 17 to the cathode of thedevice 19. The gridof the control device-"17 is" normally biasednegativ-ely meansoi a source of potential with respectito its cathode, such that sufiiclent cur this device may be controlled in response to audiocurrents receivedirom the detector -11 through the circuit 15, line amplifierltl and controlamplifier 32. These voice currents arerectified-by the device 33, thereby causing a potential to'be produced upon a resistance 34gwhich is connected between'the grid and cathode of the device 17,-having" a polarity opposite to that 'otthe battery Thus wheiraudio currents are TGQQlVGCl' from the detector 11 the grid of device 17 will become sufliciently positive to permit current to flow in theanode circuit of the in which the device l9f tlierebyto cause that: device to I produce osci-llations. This current flowing? throughthe resistance 32 will cause a ass tive potential to appearupon the grid' of electron discharge device and thereby cause operation of relay 20 andint'erruption of theoutput circuit of detector 12; f

. The adjustment and operation of thefde vices 17, 33 and 35 are' more fully describedl" in Patents No.11,'696, 566,r Emmett Carter, entitled'High frequencyduplex signaling'sys tem, issued December25', 1928; andNo. 13696,

590, Lester F. Bird,"entitled Signaling system issued December 25, 1928, andcopending ap plication Serial "No! 262,581 of William A; Tolsom ientitled Signal system, filed'March 17, 1928', allot whicha're assigned tothe same assignee as the present applicati l a Oscillationsproduced bythe device be supplied tothe an'iplifierthrough the con-- plingcondenser 30. These'currents are then amplified by device 14 and modulated by the modulating device22 in response tocurrentsreceived through the linejaInplifier 16 and a from amplifier" 14"are then supplied *toi the I winding 13 andtothelineli The circuit arrangements for controllingq the oscillation generator-26f and the relay 27 are identical with those ust described in connectionwith the oscillation generator :19

and the relay 2 0 with the exception'that voice currents for controlling thesedevices' ar'ereceived -i'ro'inthe detector 12, through the line 3 amplifier23'and control amplifier 36; Audio} currents arefsuppli'ed fromline a1npl ifier23 through speeclrampli'fier2 l to the-modulaf "As thus described it will be seen that the same coupling circuitconnectingthe repeat-: ingstation to the line, and the same high 'fre quency amplifier and'modulatingdevice are used for repeatingthe signals in bothfldirections. cc l c In Fig. 3 I} have shown a repeatingstation which is arranged to repeat signals between the stations 3 and 4:- or between any of these stations and a third station- 37 all of a which may operate at difierentfrequencies. The

station 37 may be located upon line 1 in which a case its frequency is preferably; different from that of the stations 3 or it may be lotem in which case its'fre'qiiency may be de termined in accordance with the characteris tics of that'system and requirements of the communicationchannel; a I a The repeatmg; system SllOWIilIl cated upon an entirely different power sys-- Fig. 300m? l prises a pair; of oscillation generators 38 and 39. The oscillation generator 38 is normally;

connected to supply currents through the" armatures of relay 40, in their left-hand position, to the high frequency amplifier .ll from i which they are supplied through hybrid coil 42 to the station 37. These currents will of course have the frequency to which the stations 37 are adapted to respond. The oscillation generator 39 is normally connected to supply current through the amplifier 43 and hybrid coil 44 to the line 1 having the frequency to which the stations 3 are adapted to respond.

The oscillation generator 39 is provided with an inductance 45 which may be connected-in a suitable manner to vary the frequency of the oscillation generator, as for example bybeing connected in circuit relation with the oscillatory circuit of the generator. This inductance is of such value that when so connected, as by means of a relay 46, the current produced by the oscillation generator will be of the frequency towhich the station 4 is adapted to respond. The oscillation generator 38 is similarly provided with an inductance 47 which may be connected in parallel with the oscillatory circuit of the generator by means of a relay 48. When so connected the oscillation generator 38 will produce currents having the frequency to which the stations 3 are adapted to respond. The relay 40 is arranged to be energized simultaneously with the relay 48 and in operating its contacts to the right connects the output circuit of the oscillation generator 38 to the input circuit of amplifier 43 in a manner such that these currents may be supplied through that amplifier to the line 1 and to the stations 3.

For the purpose of controlling the relays 40 and 48, which are associated with the oscillation generator 38, and the relay 46, which is associated with the oscillation generator 39, and other relays later to be indicated, in response to currents received from the stations 3, 4 and 37 suitable selectors 49 and 50 are provided. The selector 49 is adapted to respond to current flowing in the anode circuit of either of the detectors 51 or 52, the detector 51 being tuned to respond to currents of the frequency produced by the stations 3 and the detector 52 being tuned to respond to currents produced by the stations 4. The selector 5O similarly responds to current flowing in the anode circuit of a detector 53 which is tuned to respond to currents produced by the station 37.

A further understanding of the system shown in Fig. 3 may best be had by reference to its operation.

As shown the system is normally connected to repeat signals between the stations 3 and the stations 37. As such its operation is as follows. \Vhen the operator at the station 37 speaks into the microphone he will cause carrier current modulated with voice currents to be supplied to the detector 53 where it will be rectified. Voice currents resulting from this rectification will be supplied through a line amplifier 54 to a control tube 55. This control tube is arranged to control the operation of the oscillation generator 39 in the manner described in connection with Fig. 2. High frequency current having the frequency at which the station 3 is adapted to respond will then. be supplied through the radio amplifier 43 and the hybrid coil 44 to the line and hence to the stations 3. This high frequency current will be modulated with voice currents supplied from the amplifier 54 through a speech amplifier 56 to modulating device 57 which is associated with the anode circuit of the amplifier 43 in the manner described in connection with Fig. 2.

In the same way when the operator at the station 3 speaks into his microphone he will cause modulated carrier current to be supplied to the detector 51 and rectified. The output from the detector 51 which is of voice frequency will be supplied through the armature and lower contacts of relay 58, and line amplifier 59 to a control tube 60. This control tube controls the operation of the oscillation generator 38 in the manner described causing it to produce oscillations which are supplied through the armatures and left hand contacts of the relay 40 to the power amplifier 41. These high frequency currents are then amplified and modulated in response to the voice currents which are supplied thereto through amplifier 59, speech amplifier 61 and a modulator 62. This modulated high frequency current will then be supplied through the hybrid coil 42 to the stations 37.

Relay 62 is connected to respond to current flowing in the anode circuit of the oscillation generator 39 in the manner described in connection with relay 20 of Fig. 2, such that during operation of the oscillation generator 39 this relay is energized and short circuits the output from the detectors 51 and 52, thereby preventing any transmission of energy through the opposite channel and hence preventing operation of the oscillation generator 38. In the same way when the oscillation generator 38 operates the relay 63 will be energized to short circuit the channel in the opposite direction and thereby preventing operation of the oscillation generator 39.

Let us assume now that the operator at the station 4 desires to speak with the operator at the station 37. He will dial the number of the called party in the same manner in which he would call any station upon his own line but the number will be a special one such that the selector 49, in responding to the carrier impulses produced by the dialing device in the station 4, will cause a circuit 64 and relay 46 to be energized, thereby connecting the oscillation generator 39 to produce oscillaq tions having the frequency at which the station 4 operates. The repeating of signals be tween the station 4 and station 37 will then be exactly as described previously in connection with the repeating of signals between station 3 and the station 37 with the exception that the detector 52 will beoperative to r ad at Omaneem aceivea'eem sge, mm 4 and the generator 39 will produce cur rent having the frequency to Y which the sta-.

'tion4 "responds.

- "Ifit be assumed that the'operatoriat Sta-V tion37 desires to call the station ihe wlll dial a number to which the selector 50 will respond. This selector will then cause" the energizationof relay 46,causing the oscillae :ti on generator 39 to operate-at the frequency of the station The repeati'ngof signals will thenmke place as described above. V p If the operator at either of the stations .3,

or: l,"- desires tofspeakiwithihe operator at theother stationhe ,Will dial a number to which theselector 49 will respond to *cause energization of relay 65. The relayg65 will then close "both .of its contacts causing energization of relays 46, 40, 48, 58 and 66. t Energiz ation of relay 46 will cause the oscillation generator 39 to operate at the frequency at which thestation operates The operation of relay 48 willcause the oscillation genera- -tor 38 to operate at the frequency to which the station 3 responds. Operation of relay .40 will connect the output-of oscillation genferator 1 38"to' theinput circuitof amplifier- 43 and disconnect this output circuit from p Qamplifierel. The operationofrelay fifiwill tothe input of rnodulator 57 and'relay. 58: willdisconnect theoutput'ofdetector 51 frorn connect the output frorni speech amplifier 61 line amplifier 59. and connect it to the, input :As thus connected the" repeating Tnalsbetween stations3 and twill e circuit of'line-amplifiere l. l

as follows, When thegoperator at the station13 speaks. into hisf microphone, he will! cause modulated carrier currents to be impressed upon detector 51. Currents flowing'in the output circuit of detectorf5l,whichare of voice 1 frequency, will then. be supplied .bsllpplied as before tothe "controltube 55 and to the inodulating device 57 The control through the armatures and upper contacts of relay 58 to the input-circuitto the amplifier 54. The output from arnplifierb ljwill then tube 55Ycausesioperation of the "oscillation generator39andthe production of high frequency-oscillations having thefrequency to 5 which the station 4 is adapted to respond,

; .thQSQcurirents being amplified by theldevice 4:3 andmodulatedby'the device 57, .These te'ctor 52 and preventinganv transmission of rnodulated currents are supplied through'coil tcto the linel Upon operation of theoscil-M lationr generator 39 the relay .62wil1 respond thereby short circuiting the output from deenergythrough this channel such as might cause operation 'of the oscillation generator q the o eras. an... station); re-

sponds by spea ing into his microphone he will cause modulated carrier current to be supplied to the detector: 52." Audio currents frorn circuit of this'detector will then be amplified by the device 59 andsup plied to the control tube 60, thereby causing 3: are adaptedto respond, will then be supplied through circuit 67 to the input of amplifierl43; Voice currents from the ampliarnplifier 61, armatures and contacts 'of relay 66 to the modul'ator 57 where they will "be caused toinodulate currents amplified arnplifier 43. Modulated carrier urrent hav-.

,ing theflequency tofwhich thest ations 3 are adapted to respond will "then be supplied to thej1ine,, the signals being transmittedifover the line i'ni the same direction from which operation fot the oscillation generator 38. Output current from the oscillation generator 38 having the frequency to which the stations fier.59 will also'besupplied through speech 17:

they were received. 'Upon operation of the oscillation generator 38, relay 631 Will beener- 'gizedthereby short circuiting the inputto amplifier 54 andpreve'nting any transfer of energy through this channel such would cause operation: of the oscillation generator a While the. as... an sam a a-e0 ular embodimentsof my-inventionit will of 7 my invention as Saran-.11 in the app d d claims. *1 l 3 What I -as; new and desire" to. secured byLett'ers Patent of: theUnited States, is:

L1."The combinationflina carrier current communication system; ,with a plurality of power lines, intercommunrcating carrler sta- 1 time on eachline,, the stations on diflierent linesoperating at different frequencies, a carrier channel 1nterconnect1ngsa1d l nes; and jnieans locateduponone ofsa1d lines betweenstations ony'said line for receiving signals from any of said stations with a carrier" wave n having predetermined frequency and repeat ing saidsignal to'a stationona different line with a, carrier wave having the frequency at {which said last station operates wherebyfduplex communication maybe eilected between stations on difierent lines. j

2. lncombinationy a plurality of? intercommunicating carrier signaling stations, each of saidstatlons being arranged to trans.

rnit andreceive; on the sanne carrier frequency and certain of sa1dstat1ons operating on a "carrier frequency difiiere'nt irom that at which other of "said" stations operate; a line;

joining and extending beyond saidstationspl a repeatinglstation connected to'said line as extendedahd arranged to receive calling and signaling currents from anyof saidstations andto transmit said calling and signaling from which they were receiveditoanotherof currents over saidline in thesame direction 1 said stations with a carrier wave having the frequency at which said last mentioned station operates. A

3. In combination, a plurality of intercommunicating carrier stations operating at different frequencies, a repeating station, an amplifier in said repeating station, a plurality of oscillation generators operating at different frequencies arranged to supply currents to said amplifier, each of said genera-- tors operating at the frequency of one of said stations, and means responsive to current received from any of said stations for causing a generator operating at the frequency of a different station tosupply current through said amplifier to said different station, and means responsive to operation of each of said generators for preventing the supply of high frequency current to said amplifier fromvanother generator.

4. In combination, a pair of intercommunicating high frequency signaling stations operating at different frequencies, aline joining said stations, a repeating station including an amplifier connected to the line, a pair of normally inactive oscillation generators connected to supply current to said amplifier, each generator being arranged to operate at a different frequency and at the frequency of one of said intercommunicating stations, and means responsive to current received from either station for rendering one of said generators operative thereby to supply current having the frequency of the other station through said amplifier to the line.

5. In combination, a'plurality of intercommunicating carrier stations, each of said stations being arranged to transmit and receive on the same carrier frequency, and one of said stations operating at a difierent frequency from another of said stations, and a common repeating system arranged to repeat signals from either of said last mentioned stations to the other at the frequency at which said other station is adapted to operate and to repeat signals in both directions for duplex communication between either of said stations and a third station.

6. In combination, a plurality of power lines, a plurality of intercommunicating carrier stations on each line, the stations on oneline operating at a different frequency from stations on another line, a common repeating system connected to one of said lines for repeating signals in both directions over said line for duplex communication between stations associated with said line, and between any of the last mentioned stations and a station on the other line.

7. In combination, a plurality of intercommunicating carrier stations, each of said stations being arranged to transmit andreceive on the same carrier frequency, and one of said stations operating at a different frequency from another of said stations, and a common repeating system arranged to repeat signals from either of said last mentioned stations to the other at the frequency at which said other station is adapted to operate and to repeat signals from either of said stations to a third station.

8. The combination in a carrier current communication system includinga repeating station having a plurality of carrier channels extending therefrom, a pair of carrier current generators in said station, means utilizing said generators for repeating signals for two way communication between different channels and means utilizing the same generators for repeating signals for two way com- ,munication between different stations on the same channel.

9. The combination in a carrier current communication system including a repeating station having a pair of carrier channels extending therefrom, a carrier current generator normally connected to supply current to each channel, means for modulating current produced by each generator with current received from the other channel, and means responsive to currentreceived from one channel for connecting the generator which is normally connected to supply current to the other channel tosupply current to said one channel and for modulating current produced by either generator with current received from said one channel whereby repeating of signals between different channels or between differ ent stations on one channel maybe efiected.

10; In combination, three or more carrier stations, each of said stations being arranged totransmit and receive on the carrier, frequency-and on a carrier frequency different from that at which other of said stations operate, and a common repeating station arranged to receive signals from any of said stations and to repeat the received signals to any of the other stations at the carrier frequency at which said other station operates.

11. The combination in a carrier current communication system including a repeating station having a pair of carrier channels extending therefrom, a carrier station connected to one channel and a plurality of carrier stations operating at different frequencies connected to the other channel, a pair of carrier current generators, means responsive to current received from either channel for causing one ofsaid generators to supply current to the other channel having the frequency at which a station associated with said other channel operates, and means responsive to current received from either channel for adjusting the generator for supplying current to said first mentioned other channel to supply current of desired frequency to said other channel whereby duplex communication between a station on said one channel and either of a plurality of stations operating at difierent frequencies on said first mentioned otherchannel may be carried on.

12. The combination in a carriercurrent communication system including a repeating station having a pairof carrier channels extending therefrom, a carrier station connected to one channel and a plurality of car-- rier stations operating at different frequencieswconnected to the other channel, an oscillation generator normally connected to supply carrier currentto each channel having a certain frequency, 'means utilizing said generators for repeating signals inboth directions between said channels, means responsive to impulses received from the mentioned other channel for changing the frequency of both of said oscillation generators and for connecting bothgenerators to supply current to said other channel, and means re-I' sponsive to current received from either station on said other channel for modulating current produced by one of said generators whereby duplex repeating of signals between stations on said other channel may be efi'ected- In Witness whereof, Ifhave hereunto set a my hand this 13th day of Se tember, 1928.

FREDERIG i FBINK. 

